The radio-controlled transmission of time information is performed by transmitting so-called time signals by respective transmitters referred to herein as time signal transmitters. The term “time signal” is intended to mean a transmitter signal of short duration which is transmitted for providing time information in the form of a time reference provided by the transmitter. The time reference is a modulated oscillation generally comprising several time markers which, upon demodulation merely are an impulse which reproduces the transmitted time reference with a certain uncertainty or inaccuracy.
The German long wave time signal transmitter station DCF-77 transmits, in a continuous operation controlled by atomic clocks, amplitude modulated long wave time signals in accordance with the official atomic time scale MEZ (middle European time) with a transmitter power of 50 kW at a frequency of 77.5 kHz. Similar time signal transmitters exist in other countries for the transmission of time information on a long wave carrier frequency in the range between 40 to 120 kHz. All countries using such transmitters transmit the time information as a telegram having a duration of exactly 1 minute.
FIG. 1 shows an encoding scheme A referred to as a telegram. This telegram A represents the encoded time information transmitted by the German time signal transmitter DCF-77. The encoding scheme or telegram A comprises 59 bits, whereby one bit corresponds to one second of the time frame. Thus, within the duration of 1 minute a so-called time signal telegram can be transmitted. This telegram comprises binary encoded information, particularly information regarding time and date. The first fifteen bits B comprise a general encoding which, for example, may contain operational information. The next 5 bits C contain general information. Thus, the letter R designates the antenna bit. A1 designates an announcement bit for the transition from the central European time (MEZ) to a central European summer time (MESZ) and back again. Bits Z1 and Z2 designate time zone bits. Bit A2 designates an announcement bit for a leap second and bit S designates a start bit for the beginning of the encoded time information. Starting with bit 21 and up to bit 59 these bits transmit the time and data information with a BCD code, whereby the data are respectively relevant for the next following minute. The bits in the area D contain information regarding the minute. The bits in the area E contain information regarding the hour. The bits in the area F contain information regarding the calendar day. The bits in the area G contain information regarding the day of the week. The bits in the area H contain information regarding the month. The bits in the area I contain information regarding the calendar year. These informations are provided in a bit-by-bit fashion in an encoded form. So-called testing bits P1, P2, P3 are provided respectively at the ends of the areas D, E and I. The sixtieth bit of the telegram is not designated and serves for indicating the beginning of the next time frame. The letter M designates the minute marker and thus the beginning of a time signal telegram.
The structure and the bit allocation of the encoding scheme or telegram A of FIG. 1 for transmitting of time signals is generally known and described, for example in an article by Peter Hetzel, “Time Information and Normal Frequency” in the Publication “Telecom Practice”, Vol. 1, 1993.
The time signal information is transmitted with the aid of individual second markers amplitude modulated onto a carrier. The modulation comprises a reduction X1, X2 or an increase of the carrier signal X at the beginning of each second. In the case of the German time signal transmitter DCF-77 the transmitted time signals are modulated onto the carrier amplitude at the beginning of each second, with the exception of the fifty-ninth second, within each minute. For example, reducing the carrier amplitude for 0.1 second represents X1, reducing the carrier amplitude for 0.2 seconds represents X2. The amplitude reduction amounts to about 25% down from the amplitude peak. These amplitude reductions X1, X2 of different time durations define respective second markers or, after decoding, data bits in decoded form. These different time durations of the second markers serve for the binary encoding of the clock time and date. Second markers X1 of a duration of 0.1 seconds correspond to the binary “O” and time markers X2 with a duration of 0.2 seconds correspond to the binary “1”. The absence of the sixtieth second marker announces the next following minute marker. An evaluation of the time information transmitted by the time signal transmitter may then be performed in combination with the respective second.
FIG. 2 illustrates a portion of an example of an amplitude modulated time signal. However, the evaluation of the precise time and the precise date is only possible if the fifty-nine second bits of a minute are recognized unambiguously so that a logic “0” or a logic “1” may be allocated to each of these respective second markers.
Modern time signal receivers comprise a so-called automatic gain control stage (AGC). Such automatic gain control stage makes it possible to automatically adapt the amplification to a respective level of the received time signal, thereby taking into account a change of the amplitude of the received time signal. Without such automatic gain control adjustment always the same amplification would be used, which would lead to an undesirable over-control or an under-control. The automatic gain control is particularly advantageous for signals that have been damped more or less during transmission over the transmission range. Further, the automatic gain control optimizes the sensitivity of an amplifier.
A problem exists with received time signals because frequently noise signals are superimposed on the time signals. Such noise signals can, for example be caused by electromagnetic radiation of electric and electronic equipment within the transmission range of the time signal transmitter to the time signal receiver. Such noise signals may also be caused by the electric components of the time signal receiver itself. In this connection the use of an automatic gain control would follow with its amplification in an undesirable manner the time signal change caused by the noise signal. The problem is particularly seen in that depending on the type and size of the noise signal, a faulty receipt of the time signals may be involved. The term “faulty receipt” means that during the duration of a received minute protocol erroneous binary decisions are being made which lead to a faulty evaluation of at least one data bit of the minute protocol. The time derived from the received time signal would, in that case, no longer be correct.
When known electric disturbances occur, for example caused by the advancing of a stepping motor that drives the pointers of the radio-controlled clock it is possible to hold the automatic gain control to an amplification value that was present prior to the occurrence of such a disturbance. Such holding of the amplification prevents making the time signal receiver less sensitive by an undesirable tracking of the automatic amplification adjustment caused by a noise signal. This holding of the amplification to a value not disturbed by noise can, however, only be globally applied to the entire received time signal and only when the noise impulse is known. A purposeful application of this method to a particular second impulse or a particular data bit within the minute protocol is, however, not possible.
Another problem results when the protocol of the transmitted time signal provides second impulses of rather long duration. Since the time signal amplitudes are modulated, the AGC closed loop control begins the tracking of the amplification value in the receiver already when second impulses occur that are critical to the AGC control. When AGC-critical second impulses occur and taking into account the dynamic of the AGC amplifier, the second impulses can be so long that a change in the amplitude caused by the second impulse already causes a tracking of the AGC amplification. This change of the AGC amplification leads directly to the situation that at a next modulation jump, that is, at the end of the respective AGC-critical second impulse, at which the amplitude of the time signal returns to its normal value, the receiver is over-controlled due to the now present larger amplification. An unnecessary larger amplification leads to distortions of the signal in the receiver at the next second impulse because the closed loop control voltage and thus the automatic AGC adjustment is no longer at the mean value of the received time signal that is necessary for the field strength of the received time signal. This situation can lead, depending on the type of time signal received, to a reduction of the receiver sensitivity. The sensitivity reduction in turn will result in a reduced receiver range of the radio-controlled clock and such reduced range will be noticeable by the user of the clock.
A further problem is seen in that the duration of the next following second impulse can be recognized as being faulty which in turn leads to faulty interpretations that, in the worst scenario, can lead to a faulty decoding.
The above mentioned problems could be solved by influencing the closed loop control voltage. Such influencing of the closed loop control voltage and thus of the adjustable amplification can basically be accomplished in software fashion. However, the required software would be extraordinarily costly, particularly with regard to the storage capacity required for the software. Such a software solution would perform an automatic amplification adjustment only for those cases of an undesirable disturbance and in case of a change of the amplitude mean value of the time signal. Such software solution would not be effective in connection with a time signal amplitude change that is caused by a second impulse. In order to realize such a function capability with the respective dynamic, a powerful micro-controller would be required. However, currently, radio-controlled clocks are typically equipped with a four-bit micro-controller. Such a micro-controller does not provide the required dynamic for an effective influencing of the feedback control voltage or at least it would only be of limited use. More specifically, a complete implementation by software for controlling the closed loop feedback voltage is only possible with such a micro-controller to a very limited extent because the requirements exceed the capabilities of a four-bit micro-controller. Costs prohibit the use of a more powerful micro-controller in connection with radio-controlled clocks.
The following references provide a general background information for radio-controlled clocks and receiver circuits for receiving time signals. These references are: DE 198 08 431 A1, DE 43 19 946 A1, DE 43 04 321 C2, DE 42 37 112 A1 and DE 42 33 126 A1. With regard to information retrieving and information processing of time information from time signals, reference is made to the following German Patent Publications DE 195 14 031 C2, DE 37 33 965 C2, and EP 042 913 B1.